deb packaging: build-deb.sh + packaging/deb, icons from ico/

- scripts/build-deb.sh: binary built in ubuntu:22.04 (glibc 2.35 baseline),
  Depends via dpkg-shlibdeps, DEB_REVISION (0.1.28-1), changelog.Debian.gz,
  postinst/postrm cache refresh, REPO=1 local apt index, self-checks
  (version/upgrade path, stale binary, desktop-file-validate)
- packaging/deb/: control.in, rcalc.desktop, appdata metainfo, changelog, hooks
- ico/: new app icon in 32/48/64/96/256/512, used for the hicolor slots
- convert.rs: fix clippy::approx_constant error in roundtrip test (PI constant)
- README: install/update/release instructions

packaging/deb/changelog

@@ -0,0 +1,12 @@
+rcalc (0.1.28-1) unstable; urgency=medium
+
+  * Upstream 0.1.28 (version from Cargo.toml), first Debian packaging.
+  * Binary is built in an Ubuntu 22.04 container (glibc 2.35 baseline), so the
+    package installs on Ubuntu 22.04 / 24.04 / 26.04.
+  * Ships /usr/bin/rcalc, /usr/share/applications/rcalc.desktop, hicolor icons
+    taken from ico/, AppStream metainfo and postinst/postrm cache refresh hooks.
+  * Depends are generated from the ELF with dpkg-shlibdeps.
+  * Build fix: clippy::approx_constant error in the convert round-trip test
+    (3.1415927 -> std::f32::consts::PI).
+
+ -- alexMB5300 <prom-ing@yandex.ru>  Thu, 17 Sep 2026 19:12:31 +0300